File photo showing a Russian MiG-29 plane flying.

Russia has sent fighter jets to Syria, U.S. officials said, raising the stakes in a military buildup that has put Washington on edge and led Friday to the first talks between U.S. and Russian defense chiefs in over a year.

U.S. Defense Secretary Ash Carter, eyeing the possibility of rival U.S. and Russian air operations in Syria’s limited airspace, agreed in a call with his Russian counterpart to explore ways to avoid accidental military interactions.

The coordination necessary to avoid such encounters is known in military parlance as “deconfliction.”

“They agreed to further discuss mechanisms for deconfliction in Syria and the counter-ISIL campaign,” Pentagon spokesman Peter Cook said after the call, referring to the campaign by the U.S. and its allies against ISIS militants.

The former Cold War foes have a common adversary in ISIS militants in Syria, even as Washington opposes Moscow’s support for Syrian President Bashar al-Assad, seeing him as a driver in the nation’s devastating, four-and-a-half-year civil war.

A senior U.S. defense official, recounting details of the conversation, said Russia’s Defense Minister Sergei Shoigu had described Moscow’s activities in Syria as defensive in nature.

Shoigu said Russia’s military moves “were designed to honor commitments made to the Syrian government,” the U.S. official said.

It was unclear, however, what those commitments to Syria are or how Russia’s military buildup was relevant to them.

Russia’s latest deployment to Syria

Russia’s latest deployment has added significant airpower to a buildup that, according to U.S. estimates, also includes helicopter gunships, artillery and as many 500 Russian naval infantry forces at an airfield near Latakia.

One U.S. official, speaking on condition of anonymity, said four tactical Russian fighter jets were sent to Syria. Another U.S. official declined to offer a number but confirmed the presence of multiple jets.

U.S. ‘common ground’ with Russia

In London, U.S. Secretary of State John Kerry said the United States was looking to find “common ground” with Russia.

Kerry said it was important to forge a political agreement in Syria and end the hardship of Syrian people.

“Everybody is seized by the urgency. We have been all along but the migration levels and continued destruction, the danger of potential augmentation by any unilateral moves puts a high premium on diplomacy at this moment,” he said.

Still, the White House cautioned Moscow against “doubling down on Assad.”

Carter told Shoigu that future consultations would run in parallel “with diplomatic talks that would ensure a political transition in Syria,” Cook said.

“He noted that defeating (ISIS militants) and ensuring a political transition are objectives that need to be pursued at the same time,” he said.